table.cc, sql_select.cc:

  Limit the fix for bug 28591 to InnoDB only

sql/sql_select.cc

@@ -12033,6 +12033,7 @@ static int test_if_order_by_key(ORDER *order, TABLE *table, uint idx,
       */
       if (!on_primary_key &&
           (table->file->table_flags() & HA_PRIMARY_KEY_IN_READ_INDEX) &&
+          table->s->db_type == DB_TYPE_INNODB &&
           table->s->primary_key != MAX_KEY)
       {
         on_primary_key= TRUE;

sql/table.cc

@@ -782,7 +782,8 @@ int openfrm(THD *thd, const char *name, const char *alias, uint db_stat,
 	    if (ha_option & HA_PRIMARY_KEY_IN_READ_INDEX)
             {
 	      field->part_of_key= share->keys_in_use;
-              if (field->part_of_sortkey.is_set(key))
+              if (share->db_type == DB_TYPE_INNODB && 
+                  field->part_of_sortkey.is_set(key))
                 field->part_of_sortkey= share->keys_in_use;
             }
 	  }